The present invention relates to a control system for a motor adapted to drive for repeated reciprocation a carriage of a copying apparatus carrying a lamp, mirrors etc., a scanning head of a facsimile apparatus, a recording head of a printer or like movable body.
In a control of a motor of the type described, it is desirable that the motor settles at a constant velocity as quickly as possible after a start of its movement. A motor drive control system heretofore known is designed to perform a constant velocity control employing a phased locked loop or PLL. The phase locked loop control system usually comprises a rotary encoder connected with a motor, a phase comparator, a loop filter and a motor drive circuit which includes a servo amplifier. However, such a control system requires a substantial period of time for the motor to enter a constant velocity control mode from a standstill or from an instant of reversal. This originates from the fact that due to the presence of the loop filter a time period corresponding to the time constant of the loop filter is necessary before the motor enters a constant velocity movement. It has been customary to overcome this drawback by supplying a given level of voltage to the motor drive circuit as an energizing signal at a start of motor operation while, during this period of time, charging the loop filter so that its output voltage reaches a constant velocity indication level for a constant velocity control (as disclosed in Japanese Layed Open Patent Application No. 54-35312/1979). As the motor is accelerated up to a predetermined velocity or as a predetermined energizing time for a start of the motor expires, the loop filter is discharged to switchingly supply the motor drive circuit with its output voltage. This type of prior art control system is advantageous in that the time period the motor takes to reach a predetermined velocity is short, in that the motor can enter the constant velocity control mode without any substantial hunting, and in that the buildup time necessary for the motor to settle in the constant velocity mode after a start is short.
However, the prior art control system described above cannot avoid intricacy of construction. For instance, where a plurality of motor velocities can be specified, that is, where pulses synchronous with actual rotation of a motor are to be compared with reference pulses having a period which corresponds to a specific motor velocity, the voltage at the loop filter must reach a level corresponding to the specific motor velocity before the motor enters a constant velocity control mode. The control system therefore needs initial voltage setting circuits equal in number to the designatable steps of motor velocity and installed in the loop filter section or its input section or between the loop filter and the motor drive circuit, and a selector circuit for selecting one of the initial voltage setting circuits each time.
Meanwhile, where the frictional forces in a scanning mechanism driven by the motor are relatively large (and/or the loop gain is relatively small), the motor has to exert a large torque. If an arrangement is made such that a current larger than a certain value can be fed to the motor in a constant velocity control mode, the phase difference will be stabilized at a value corresponding to the frictional forces while the output voltage of the loop filter will settle at a given level on the motor accelerating side with respect to the zero level. However, a problem is that the motor is decelerated temporarily upon a shift from a start mode to a constant velocity control mode and, then, accelerated until the output of the loop filter reaches the given level on the motor accelerating side, which results in a temporary oscillation of the motor and, therefore, the scanning mechanism.